feat: Implement multi-touch drag and long-press stack selection with pointer tracking and physics model documentation.

frontend/src/desktop/LayoutSystem.ts

@@ -32,6 +32,7 @@ export class LayoutCard implements LayoutCardState {
     public intendedX: number = 0;
     public intendedY: number = 0;
     private dragOffset: { x: number, y: number } | null = null;
+    public dragPointerId: number | null = null;
 
     constructor(id: string, store: LayoutStore, initialData: Partial<LayoutCardState> = {}, ref: HTMLElement | null = null) {
         this.id = id;
@@ -49,10 +50,11 @@ export class LayoutCard implements LayoutCardState {
         this.applyTransform();
     }
 
-    beginDrag(offset: { x: number, y: number }) {
+    beginDrag(offset: { x: number, y: number }, pointerId: number) {
         this.dragOffset = offset;
         this.dragStartX = this.x;
         this.dragStartY = this.y;
+        this.dragPointerId = pointerId;
     }
 
     getConstrainedPosition(x: number, y: number): { x: number, y: number } {
@@ -91,6 +93,7 @@ export class LayoutCard implements LayoutCardState {
 
     finishDrag() {
         this.dragOffset = null;
+        this.dragPointerId = null;
     }
 
     update(changes: Partial<LayoutCardState>, options: { markDirty?: boolean, isConstraintUpdate?: boolean } = {}) {
@@ -278,6 +281,10 @@ export class LayoutCard implements LayoutCardState {
     get centerY(): number {
         return this._centerY;
     }
+
+    get isDragging(): boolean {
+        return !!this.dragOffset;
+    }
 }
 
 export class LayoutStore {